What was found
AIR TURN BACK DUE TO CONSECUTIVE 'LOUD BOOMS' FOLLOWED BY THE AIRCRAFT SHAKING VIOLENTLY DURING CLIMB WHILE BRINGING THE LANDING GEAR UP. IT DID NOT CONTINUE, SO THE CREW FLEW FOR ABOUT 5 MINUTES AS THEY TRIED TO DETERMINE IF IT WAS THE ENGINE OR IF IT WAS A LANDING GEAR ISSUE. THEY RECEIVED NO ERROR MESSAGES ON THEIR ICAS, BUT DECIDED TO DECLARE EMERGENCY AS A PRECAUTION AND TURN BACK. CREW BELIEVES IT WAS A COMPRESSOR STALL. NO VISIBLE DAMAGE TO THE AIRCRAFT. AIRCRAFT LANDED WITHOUT INCIDENT AT KBFI.